1. Field of the Invention
This invention relates to an osteosynthesis device comprising a rod, and a member having a threaded section extending from a head and fitted with a nut.
2. Description of the Prior Art
Such a known osteosynthesis device for stabilizing and correcting the spine is fitted with at least two screws, as a member with a threaded section. In addition to the threaded section serving to connect each of the screws to the consolidation rod by means of the nut, the screw comprises a partially or completely threaded portion to be implanted most often in the pedicle of a vertebra.
The consolidation rod extends along the spinous apophysis of the spine and must be connected quickly to the threaded sections of plural pedicular screws, while ensuring an accurate positioning of the screws in relation to one another and especially an efficient dismountable connection between the screws and the rod.
According to the prior art, the rod is connected to a screw by tightening the rod between two adequate nuts screwed onto the threaded screw section and laterally to the latter. According to another known embodiment, a portion of the rod is wedged between a washer having a half-gutter-shaped rim and a bevelled edge of the nut; the nut pushes the rod onto the washer and the washer against the screw head separating the threaded section and the implanted screw portion.
These known connections between screws and rod do not provide a sufficient bearing area for the nuts, or for the half-gutter-shaped washer and the nut, on the rod to prevent rotation, or even translation, of the rod. The threaded section and particularly the nuts undergo asymmetrical stresses about their axis, thereby increasing the risk of breakage of these parts, notably when the consolidation rod is subjected to a bending or twisting of the spine.